Fibre optic sensors are becoming a well-established technology for a range of applications, for example geophysical applications. Fibre optic sensors can take a variety of forms, and a commonly adopted form is to arrange a coil of fibre around a mandrel. Point sensors such as geophones or hydrophones can be made in this way, to detect acoustic and seismic data at a point, and large arrays of such point sensors can be multiplexed together using fibre optic connecting cables, to form an all fibre optic system. Passive multiplexing can be achieved entirely optically, and a an advantage is that no electrical connections are required, which has great benefit in harsh environments where electrical equipment is easily damaged.
Fibre optic sensors have found application in downhole monitoring, and it is known to provide an array of geophones in or around a well to detect seismic signals with the aim of better understanding the local geological conditions and extraction process. A problem with such an approach is that geophones tend to be relatively large and so installation downhole is difficult. In addition geophones tend to have limited dynamic range.
WO 2005/033465 describes a system of downhole acoustic monitoring using a fibre having a number of periodic refractive index perturbations, for example Bragg gratings. Acoustic data is retrieved by portions of the fibre and used to monitor downhole conditions during operation.
Fracturing is an important process during the formation of some oil or gas wells, referred to as unconventional wells, to stimulate the flow of oil or gas from a rock formation. Typically a borehole is drilled to the rock formation and lined with a casing. The outside of the casing may be filled with cement so as to prevent contamination of aquifers etc. when flow starts. In unconventional wells the rock formation may require fracturing in order to stimulate the flow. Typically this is achieved by a two-stage process of perforation followed by hydraulic fracturing. Perforation involve firing a series of perforation charges, i.e. shaped charges, from within the casing that create perforations through the casing and cement that extend into the rock formation. Once perforation is complete the rock is fractured by pumping a fluid, such as water, down the well under high pressure. This fluid is therefore forced into the perforations and, when sufficient pressure is reached, causes fracturing of the rock. A solid particulate, such as sand, is typically added to the fluid to lodge in the fractures that are formed and keep them open. Such a solid particulate is referred to as proppant. The well may be perforated in a series of sections, starting with the furthest section of well from the well head. Thus when a section of well has been perforated it may be blocked off by a blanking plug whilst the next section of well is perforated and fractured.
The fracturing process is a key step in unconventional well formation and it is the fracturing process that effectively determines the efficiency of the well. However control and monitoring of the fracture process is very difficult. The amount of fluid and proppant and flow rate are generally measured to help determine when sufficient fracturing may have occurred and also to identify potential problems in the fracturing process.
One possible problem, known as proppant wash-out, occurs when the cement surrounding the casing has failed and the fluid is simply flowing into a void. This wastes proppant fluid and prevents effective fracturing. A high flow rate or sudden increase in flow rate may be indicative of proppant wash-out.
Another problem relates to a situation that can develop where most of the fluid and proppant flows to the rock formation via one or more perforations, preventing effective fracturing via other perforation sites. Typically a fracturing process is performed for a segment of the well and, as mentioned above, several perforations may be made along the length of that well section such that the subsequent hydraulic fracturing process causes fracturing at a number of different locations along that section of well. During the hydraulic fracturing process however it is possible that the rock at one or more perforation sites may fracture more readily than at the remaining perforations. In this case one or more of the developing fractures may start to take the majority of the fluid and proppant, reducing the pressure at the other perforation sites. This can result in reduced fracturing at the other perforation sites. Increasing the flow rate of fluid and proppant may simply lead to increased fracturing at the first peroration site which may simply enlarge the fracture and not have a significant impact on how much oil or gas is received via that fracture. However reduced fracturing at the other sites can reduce the amount of oil and gas received via those sites, thus negatively impacting on the efficiency of the well as a whole. For example suppose that a section of well is perforated at four different locations for subsequent fracturing. If during the fracturing process three of the perforation sites fracture relatively readily then more of the fluid and proppant will flow to these sites. This may prevent the fourth fracture site from ever developing sufficient pressure to effectively fracture with the result that only three fractures extend into the rock formation to provide a path for flow. Thus the efficiency of this section of the well is only 75% of what would ideally be expected.
If such a situation is suspected additional, larger solid material can be added to the fluid, typically balls of solid material of a particular size or range of sizes. The size of the balls is such that they can flow into relatively large fractures where they will be embedded to cause an obstruction but are large enough not to interfere with relatively small fractures. In this way relatively large fractures, which may be consuming most of the fracture fluid, are partially blocked during the hydraulic fracture process, with the result that the flow to all fractures is evened out.
Conventionally the flow conditions of the fracture fluid is monitored to try to determine if one or more fracture sites are becoming dominant and thus preventing effective fracturing at one or more other fracture sites but this is difficult to do and often relies on the experience of the well engineers.
As well as the problems noted above merely controlling the fracture process to ensure that a desired extent of fracturing has occurred is difficult. Further, there may be more than one oil well provided to extract the oil or gas from the rock formation. When creating a new well the fractures should not extend into an area of the rock formation which is already supplying an existing well as any flow at the new well from such area may simply reduce the flow at the existing well. Determining the direction and extent of the fractures is very difficult however.
In addition to monitoring the flow rate of the fluid, sensor readings may be acquired during the fracturing process from sensors located in a separate observation well and/or at ground level. These sensors may include geophones or other seismic sensors deployed to record seismic event during the fracture process. These sensor readings can then be analysed after the fracturing process in order to try to determine the general location and extent of fracturing but offer little use for real time control of the fracturing process.